Electrostatic printing with powder applied to screen on printing side



E /B 52 VACUUM POTENTSAL su pw SOURCE souRcE.

SOURCE Sep 1967 c.o. CHILDRESS ETAL 3,340,303 ELECTROSTATIC PRINTINGWITH POWDER APPLIED TO SCREEN ON PRINTING SIDE Filed- May 19, 1966 2Sheets-Sheefl SOURCE OF POTENT\AL 4 SOURCE OF POTENTlAL.

C. O. CHILDRESS ETAL Sept. 12, 1967 3,340,803

ELECTROSTATIC PRINTING WITH POWDER APPLIED TO SCREEN ON PRINTING SIDE 2Sheets-Sheet 5;

Filed May 19, 1966 VACLU UM 50M RCE SOU RCE OF POTEN'UAL FLUID\ZEDPOWDER SLAPMTY FLUID)ZED POWDER 5U PPLY sou RCE OF POTENT\AL l THLRDSOURCE OF POTENTMNL SEcoH D sou (26E OF POTENTlAL F R 5T 50M PCE OF-POTENT \AL POTEN'HA 6 x 5 8 m y R OR E 0 D N I?! T NM. R mcA w 0 A E 4.FL DN m om MM I ET i 5y 00 B 5 O m .r am O O %P l 4 WY fi a SP n u rmwww.w .K U 0 mp Patented Sept. 12, 1967 ABSTRACT OF THE DISCLOSURE p In anelectrostatic printing system using an image screen through which powderparticles, smaller than the openings of the screen are passed into anelectric field which carries them in the form of a powder image to asubstrate, there is provided a means, at a powder loading location, forapplying powder particles to the back side of the screen. The powderparticles adhere to the screen in sufiicient number to provide anexcellent print when at a printing location, spaced from the powderapplying location, a wiper blade applied to the backside of the screenurges the powder particles through the screen openings into an electricfield to be moved to the substrate spaced from the screen. The wiperblade may be made of a material so that it may be suitably electricallybiased whereby it can control the amount of powder which is transferredfrom the mechanically wiped screen.

This invention relates to systems for printing by introducingelectroscopic powder particles into an electric field, and moreparticularly to improvements therein.

One of the present systems for printing with electroscopic powderparticles employs a screen having all openings blocked off except thosewhich provide a desired image pattern. A conductive plate is spaced fromthis screen and the substrate upon which it is desired to print, isinserted between the conductive plate and the screen surface. Theconductive plate may be eliminated if the substrate is sufficientlyconductive. A potential for the purpose of establishing an electricfield is established between the screen and the conductive plate.Electroscopic powder particles are urged through the screen aperturesinto the field between the screen and the conductive plate, whereuponthey are carried in the pattern of the image apertures of the screen tothe substrate. The substrate is thereafter removed and the image isfixed to' the substrate by well known means, suchas heatQThis generaltechnique is described and claimed in a patent to Childress et al.,Patent No. 3,081,698.

Various arrangements have been contemplated and used for moving powderparticles through the openings of the screen into the electric field. Inone of these, powder is applied to a brush which is rubbed against thescreen to flick the powder particles through the screen openings.Another technique establishes a highly charged powder cloud which isdirected at the backside of the screen by an electric field having asufiicient energy to cause the powder particles to move through thescreen apertures which open into the field between the screen and thesubstrate. It has been found desirable to print on cylindrical objectsas well as on relatively flat surfaces. When a fiat screen is used asthe image forming member for a cylindrical object, it is necssary toconfine the area of powder transfer to a relatively narrow portion ofthe screen adjacent to which the surface of the cylindrical object,which is caused to revolve, is in closest proximity. In this manner, asubstantially parallel electric field can be established between the twoproximal surface areas of the screen and cylinder. It has been founddifficult to confine the action of the brush or the action of the airsuspended particles to such a restricted area of the screen,particularly if high printing speeds are to be achieved.

An object of this invention is the provision of a novel powder loadingand powder transferring system in an electrostatic printing system ofthe type described.

Still another object of this invention is the provision of a powderloading and powder transferring system in a system of the typedescribed, suitable for high speed printing on cylindrical objects.

Yet another object of the present invention is the provision of a powderloading and powder transferring arrangement in a system of the typedescribed, which conserves the amount of powder which is used.

These and other objects of the present invention are achieved in anarrangement whereby a thin coating of electroscopic powder particles ispreloaded or caused to adhere electrostatically to the rear surface ofan image forming screen. The powder particles can then be caused to passthrough the image openings by passing a narrow wiper blade along therear surface of the screen, as a result of which powder penetrationthrough the screen is confined substantially to the contact area of theblade. Such a blade may be positioned to move along the rear of thescreen synchronously with the closest portion of the cylinder which isbeing printed. This cylinder can be transported and rotated along theflat surface of the image screen in a manner to have a minimal relativesurface velocity in the area of powder transfer. Thus, the screen may bestationary, its back surface may be loaded with powder particles, andthereafter the cylinder is moved linearly and rotated adjacent the frontsurface of the screen with a blade being simultaneously moved along theback surface of the screen. Alternatively, the screen may be moved withits back surface being preloaded with powder at one location, and at asecond location a wiper blade is positioned at the backside of thescreen with the rotating cylindrical member being positioned adjacentthe front side of the screen and opposite the blade location.

The novel features that are considered characteristic of this inventionare set forth with particularly. in the appended claims. The inventionitself both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe following description when read in connection with the accompanyingdrawings, in which:

FIGURE 1 is a schematic illustration of an embodiment of the invention;

FIGURE 2 is an enlarged representation of the section of FIGURE 1 atwhich the Wiper blade is positioned to illustrate the operation of theblade at this location;

FIGURE 3 illustrates another arrangement for preloading the screen inaccordance with this invention;

FIGURE 4 shows another arrangement for cleaning the screen front surfacewhile its back surface is being loaded; and

FIGURE 5 illustrates an arrangement for employing the invention in acontinuous belt printing arrangement.

FIGURE 1 is a schematic representation of am embodimentof the inventionusing a moving screen with the powder preloading apparatus and theobject receiving the printing moving synchronously so that it and thescreen are relatively stationary. It should be appreciated however thatthis arrangement is to be considered as illustrative since it isbelieved obvious to maintain the screen stationary and move the powderloading apparatus and the powder image receiving object past the screen,where the circumstances of manufacture require this kind of anoperation. In accordance with this invention, an image aperture screen10 moves in the direction shown by the arrows so that initially, aportion thereof passes adjacent a rear loading electrode 12. Thiselectrode comprises a conductive plate which may be spaced on the orderof one-half inch from the screen 10. A source of fluidized powder 14, orpowder supply source applies the electroscopic powder particles to beemployed in the printing through a nozzle 16 to the space between therear loading elect-rode 12 and the screen 10. A source of potential 18is connected with its negative terminal to the rear loading electrodeand its positive terminal grounded. The screen is also connected toground. As a result, an electric field is established between theelectrode 12 and the screen whereby powder in the fluidized powderstream directed downward by the nozzle 16 between the screen and theelectrode, is directed onto the screen. The location at which theelectrode 12 directs powder onto the screen may be considered as apowderloading location.

Any powder in the fluidized stream which passes completely between theelectrode 12 and the screen 10 and is not captured by the screen, iscaught in a container 20 which is below the screen. As the screencontinues to move in the direction of the arrow, the front surfacethereof, which is the surface opposite to the one on which powder isloaded, passes by a rotating belt 22, which serves the function ofwiping powder from the front surface of the screen that may havepenetrated the image openings and been deposited on the front surface ofthe screen. The belt 22 which is supported on spaced rollers 24, 26passes by the nozzle 28 which is connected to a vacuum source 30 wherebythe powder picked up by the belt 22 is cleaned therefrom.

, The screen next moves to the location of a wiper blade 32, whichserves the function of wiping the powder deposited on the back surfaceof the screen into and through the openings in the screen. Positionedirectly opposite the wiper blade is the object upon which it is desiredt print. Here the cylindrical object 34 is shown as a bottle which isrotated in the direction of the curved arrow by any suitable means, tomaintain a substantially zero relative motion with respect to the screenat the location of the wiper 32.

The wiper blade may be fabricated from a number of materials, dependingupon the density of image deposit required and the nature of theelectroscopic powder. For example, blades have been made of a thinceramic material and also of a plastic. In some cases the wiper bladehas been in effect a very narrow brush. It is desirable to apply apotential to the wiper 32. This may be done by embedding conductivematerial such as a wire 36, represented by the dotted lines, in therelatively nonconducting blade near the end closest to the screen. Thiswire is connected to another source of potential 38, which has itsnegative terminal connected to the wire 36 and the positive terminalgrounded. Assuming that the object 34 which is receiving the printing isconductive, in whch event the conductive plate behind it is omitted,connection is made thereto from the positive terminal of a potentialsource 40, which has its negative terminal grounded. By way ofillustration, but not by way of .a limitation, in an embodiment of theinvention which was built and operated, the source of potential 18 wason the order of 12 kilovolts, the source of potential 38 was on theorder of kilovolts, and the source of potential 40 was on the order of+2 kilovolts.

The method of loading the rear screen surface which is shown in FIGURE 1is particularly effective when a high density flow of powder is appliedbetween the rear of the screen and the electrode 12. High powder densitycauses the powder coating to quickly bridge the screen openings so thata minimal amount of powder penetrates these openings ahead of the wiperblade station. This may be seen by referring to FIGURE 2 which is anenlarged cross-sectional representation of the screen showing how thepowder particles 44 bridge the screen openings. The powder also adheresto the back of the screen in the region where the openings are blockedas by the member 46.

Another method for filling the openings of the screen would be to extendthe nozzle 16 so that it incorporates the electrode 12 instead ofterminating in the space between the electrode and the screen, whereby alow pressure spray nozzle with the electrode therein for establishing asuitable field, directs powder at the rear screen surface.

FIGURE 2 also shows the operation of the wiper blade 32 to push thepowder particles from the back surface of the screen through theopenings of the screen whereby the electric field established betweenthe screen and the object 34 carries these powder particles to thesurface of the cylindrical object 34.

When the wiper blade is of a rigid material, the amount of powdertransferred through the screen is somewhat dependent upon the shape ofthe leading edge of the blade and the width of the blade. By way ofillustration, and not by way of limitation, a typical blade, of an inchwide, has its end terminating at the screen in the shape shown in FIGURE2. The leading edge of the blade clears the screen by approximately ,434of an inch.

FIGURE 3 shows another arrangement for coating the back side of thescreen, in accordance with this invention. The coating arrangementcomprises a housing 50, with insulating walls except that a conductiveplate 52 is mounted in the back wall thereof. This conductive plate isconnected to the negative terminal of a first source of potential whosepositive terminal is grounded. Spaced from the conductive plate 52 areconductive wires or rods 56 which may be supported within the oppositewalls of the housing to form an open grid. A fluidized powder supplysource 58 furnishes fluidized powder to the region between theconductive plate 52 and the wires 56. A sec ond source of potential 60has its negative terminal Connected to all of the wires in the grid 56and its positive terminal grounded. The potential from the source 60 isless negative than the potential from the source 54, as a result ofwhich powder particles in the fluidized powder supply which are directedbetween the plate 52 and the grid 56, are directed through the grid 56at the periphery of a roller 62 which is revolved against the surface ofthe screen 10. As a matter of fact, the roller is rotatably supported,by any suitable means, not shown, with its periphery in frictionalengagement with the surface of the screen so that as the screen movespast the roller it causes it to rotate.

The roller is either entirely made of resistive material, or has itssurface made of a material which is sufiiciently resistive to maintain apotential difference between it and the screen with which it is incontact. The inner surface of the roller is connected with the negativeterminal of a third source of potential, which is less negative than thesecond source of potential, and which source of potential has itspositive terminal grounded. The roller may be made of any plasticmaterial or semiconductor material, for example. The use of a materialfor the roller, or a coating for the roller, which has a very limitedcon ductivity, makes it possible to establish and maintain a suitablepotential diflerence between the roller and screen so that the powderlayer which is caused to adhere to the surface of the roller by passingthrough the grid 56, is thereafter caused to transfer from the roller tothe rear screen surface under the influence of the potential from thethird source. Thereafter, the operation of the system is as has beendescribed in connection with FIGURE 1 and FIGURE 2. Accordingly, therehas been omitted the showing in this drawing of the wiper blade and thecleaning apparatus at the front side of the screen.

As described above, during the process of coating the rear of thescreen, some of the powder may penetrate the image openings and, in theabsence of any further transferring fields at this location, this powderre-attaches itself to the front screen surface surrounding the imageareas, from which location powder particles can subsequently transfer tothe'image receiving object. As shown in FIGURE 1, a wiping surface isprovided for the front side of the screen which is located between thepowder loading station and the powder transfer station of the screen.Another effective means for preventing powder from adhering to the frontface of the screen is shown in FIGURE 4. Here the structure for loadingthe back sur face of the screen which is shown is the same as that shownin FIGURE 1 and therefore the same reference numerals are applied.

On the front side of the screen opposite to the powder loading location,there is positioned a grid electrode 70, behind which there is placed anozzle 72 which is coupled to a vacuum source 74. The grid electrode isbiased positive from a source of potential 76 which has its negativeterminal grounded. Thus, any powder particles which find their waythrough the screen openings, before the screen openings are clogged orblocked by the powder accumulation, are attracted by the electric fieldbetween the screen and the grid electrode 70. These powder particlestherefore do not redeposit on the front surface of the screen but aredrawn into the vacuum source 74. Alternative to the stationary grid 70,a conductive belt may be employed which may be biased similarly toattract thereto powder particles which find their way to the openings inthe screen. The belt can be made rotatable to pass by a vacuumingdevice, in the manner shown in FIGURE 1.

FIGURE 5 illustrates a system in accordance with this invention whichmay be used with a continuous belt printer. The image screen 80 has theform of a continuous loop which is supported between two spaced rollersrespectively 82, 84. Roller 84 is driven rotatably by a motor 86. Thebelt 80 is connected to ground. The powder loading on the rear surfaceof the screen may be accomplished by any of the arrangements describedpreviously. However, still another arrangement is shown here. A hollowcontainer 88 is positioned with one open side adjacent the backside ofthe image electrode 80. A porous metal plate 90 is supported spaced fromthe bottom side of the container 88 and air from a low, positivepressure air supply source 92 is fed between the bottom side of thecontainer 88 and the porous plate 90. A powder charge is deposited overthe porous plate 90 so that under the influence of the low, positivepressure air, the powder is effectively fluidized.

Within the container and adjacent the porous metal plate there issupported rotatably a brush 94, which is rotated by any suit-able means,not shown. The ends of the brush opposite the fluidized powder bed rubagainst the wires of a grid 96, which establish a powder cloud betweenthe grid wires and the screen 80. The grid wires are connected to thenegative terminal of a source of potential 98 which has its positiveterminal grounded. The porous plate 90 is connected to the negativeterminal of a source of potential 100, whose positive terminal isgrounded. The relative amplitudes of these potential sources is suchthat the powder particles are urged from the plate 90 toward the grid 96and by the field established between the grid and the screen, toward thescreen. As the screen continues to rotate past the powder loadingstation, it is brought to the powder transfer station where the wiperblade 102 is positioned with one edge in contact with the screen rearsurface. A source of potential 104 may be connected to the wiper blade102.

The image receiving substrate here is exemplified by a web of paper 106,which is fed from a pay out roll 108, over a rotating conductivecylinder 110, which serves to move the surface of the paper close to thefront side of the screen opposite the wiper blade 102, and thereafterthrough a powder fixing station 112, which fixes the powder by heat, andonto a take up roll 114. The positive terminal of a potential source 116is connected to the conductive roller 110 and its negative terminal isconnected to ground. As a result, the potential source 116 establishesan electric field between the conductive roller and the image screenwhereby powder particles which are forced into the field by the wiperblade 102 are transferred across to the paper. The paper is movedsynchronously with the surface of the belt so that the relative motionat the nip of the roller and the image screen, at the location of thewiper blade, is substantially zero.

Any excess powder on the back surface of the screen, which is not forcedthrough the holes in the screen, falls as a result of being loosened bythe wiper blade into a trough 118, which leads back to the porous plate90, whereby the excess powder is returned to the fluidized bed to bere-used.

It should be appreciated that while the arrangement shown is one whereina printing is made from a flat screen onto an object having acylindrical surface, the advantages of this system are also to be hadwhen printing from a curved screen onto a flat object, and in some casesfrom a flat screen onto a flat surface. This arises from the fact thatin the use of a continuous belt screen for printing onto a paper web, itis often difficult to maintain a fixed gap between the image electrodebelt and the paper web. By using a wipe transfer, which is limited to avery narrow transverse section of a moving image belt, it is possible tofeed the paper around a cylindrical roller, as shown in FIGURE 5, tomore easily maintain a substantially uniform gap in the area where thepowder transfer actually takes place.

Another advantage of the above-described wipe transfer system, as thissystem may be designated, is the elimination of the ghosting problem.Ghosts are image density variations caused by depletion of the powderedink on the brush or other powder applying mechanisms, which arise as aresult of there being image areas of great demand which are incompletelysatisfied and as a result of the accumulation of ink in areas of lesserdemand. Such problems are common to roller inking systems in wet ink aswell as in dry ink printing. The wipe transfer method insures that allimage areas have been loaded with suflicient powder for a full densityimage. The metering action of the wipe blade is governed by the shape ofits edge as well as the potential applied thereto, and is largelyunaffected by excess powder on the screen which falls away from theblade so that excess powder is not accumulated.

What is claimed is:

1. In an electrostatic system a conductive screen having aperturestherethrough arranged in the form of a desired image, a screen loadinglocation, a screen unloading location displaced from said screen loadinglocation, means for relatively moving said screen between said screenloading location and said screen unloading location, said screen havinga back side to which powder particles are applied and an opposite side,said screen loading location including a source of electroscopic pow derparticles, each of said powder particles being smaller than theapertures of said screen, means for. transferring powder from saidsource onto the back side of said screen, means positioned between thescreen loading and unloading locations for removing any powder from theopposite side of said screen, said screen unloading location includingmeans for establishing an electric field between said screen and animage receiving object posi tioned adjacent one surface of said screen,and wiper means in contact with the surface of the back side of saidscreen opposite the position of said object, said wiper means wiping thesurface of said screen for pushing the powder particles on said screenthrough the apertures of said screen into the electric field betweensaid screen and object to be moved toward said image receiving objectunder the influence of said electric field.

2. In an electrostatic printing system as recited in claim 1 whereinsaid wiping means comprises a wiper blade made of resistive material andthere is included a source of potential and means connecting said source)f potential between said screen and said wiper blade for controllingthe amount of powder particles which are .ransferred from said screen tosaid object.

3. In an electrostatic printing system as recited in claim 1 whereinsaid wiper means comprises a wiper blade having a conductor adjacent'thetip thereof, said conductor being coextensive with said screen, saidmeans for establishing an electric field between said screen and saidobject includes a source of electric potential, and means for connectingsaid conductor and said object to said source of electric potential.

4. Apparatus as recited in claim 1 wherein said source of electroscopicpowder particles comprises means for generating a cloud of electroscopicpowder particles positioned at one side of said screen, and said meansfor transferring powder particles from said source onto said screenincludes a source of electric potential, and means for connecting saidsource of electric potential between said cloud forming means and saidscreen for establishing an electric field therebetween to transferpowder particles from said means for establishing a cloud of powderparticles to said screen.

5. In an electrostatic printing system as recited in claim 1 whereinthere is positioned between said screen loading location and said screenunloading location and on the same side of said screen as said imagereceiving object means for removing powder particles on that side ofsaid screen including means for wiping said screen.

6. In an electrostatic printing system as recited in claim 1 whereinthere is positioned at said screen loading location adjacent the surfaceof said screen opposite to the one 'onto which powder is loadedelectrode means, means for establishing a' potential between said screenand said electrode means for directing toward said electrode meanspowder particles which have passed through the apertures of said screen,and means for removing said powder particles from said electrode means.

7. In an electrostatic printing system as recited in claim 1 whereinsaid means for transferring powder from said source onto said screenincludes roller means rotatably supported with a portion of itsperiphery in contact with said screen, electrode means positioned spacedfrom another portion of the periphery of said roller means, means forintroducing a cloud of electroscopic powder particles from said sourcebetween said electrode means and said roller means periphery, means forestablishing an electric field between said electrode means and saidroller means periphery for transferring powder particles from saidpowder cloud to said roller means periphery, and means for establishinga potential difference across said roller means for transferring powderparticles from said roller means to said screen.

References Cited UNITED STATES PATENTS 2,484,671 10/1949 Bauman 1013,220,833 11/1965 McFarane 101 3,245,341 4/1966 Childress et a1 101--1223,251,685 5/1966 Bickmore 101 3,273,496 9/1966 Melmon 101--144 ROBERT E.PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner.

1. IN AN ELECTROSTATIC SYSTEM A CONDUCTIVE SCREEN HAVING APERTURESTHERETHROUGH ARRANGED IN THE FORM OF A DESIRED IMAGE, A SCREEN LOADINGLOCATION, A SCREEN UNLOADING LOCATION DISPLACED FROM SAID SCREEN LOADINGLOCATION, MEANS FOR RELATIVELY MOVING SAID SCREEN BETWEEN SAID SCREENLOADING LOCATION AND SAID SCREEN UNLOADING LOCATION, SAID SCREEN HAVINGA BACK SIDE TO WHICH POWDER PARTICLES ARE APPLIED AND AN OPPOSITE SIDE,SAID SCREEN LOADING LOCATION INCLUDING A SOURCE OF ELECTROSCOPIC POWDERPARTICLES, EACH OF SAID POWDER PARTICLES BEING SMALLER THAN THEAPERTURES OF SAID SCREEN, MEANS FOR TRANSFERRING POWDER FROM SAID SOURCEONTO THE BACK SIDE OF SAID SCREEN, MEANS POSITIONED BETWEEN THE SCREENLOADING AND UNLOADING LOCATIONS FOR REMOVING ANY POWDER FROM THEOPPOSITE SIDE OF SAID SCREEN, SAID SCREEN UNLOADING LOCATION INCLUDINGMEANS FOR ESTABLISHING AN ELECTRIC FIELD BETWEEN SAID SCREEN AND ANIMAGE RECEIVING OBJECT POSITIONED ADJACENT ONE SURFACE OF SAID SCREEN,AND WIPER MEANS IN CONTACT WITH THE SURFAE OF THE BACK SIDE OF SAIDSCREEN OPPOSITE THE POSITION OF SAID OBJECT, SAID WIPER MEANS WIPING THESURFACE OF SAID SCREEN FOR PUSHING THE POWDER PARTICLES ON SAID SCREENTHROUGH THE APERTURES OF SAID SCREEN INTO THE ELECTRIC FIELD BETWEENSAID SCREEN AND OBJECT TO BE MOVED TOWARDS SAID IMAGE RECEIVING OBJECTUNDER THE INFLUENCE OF SAID ELECTRIC FIELD.